Thermal management for solid state high-power electronics

ABSTRACT

The invention is for an apparatus and method for removal of waste heat from heat-generating components including high-power solid-state analog electronics such as being developed for hybrid-electric vehicles, solid-state digital electronics, light-emitting diodes for solid-state lighting, semiconductor laser diodes, photo-voltaic cells, anodes for x-ray tubes, and solids-state laser crystals. Liquid coolant is flowed in one or more closed channels having a substantially constant radius of curvature. Suitable coolants include liquid metals and liquids with low vapor pressure. The former may be flowed by magneto-hydrodynamic effect or by electromagnetic induction. The latter may be flowed by magnetic forces. Alternatively, an arbitrary liquid coolant may be used and flowed by an impeller operated by electromagnetic induction or by magnetic forces. The coolant may be flowed at very high velocity to produce very high heat transfer rates and allow for heat removal at very high flux.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional patentapplications U.S. Ser. No. 61/463,040, filed on Feb. 12, 2011 andentitled “Thermal Management for Solid State High-Power Electronics,”and U.S. Ser. No. 61/463,210, filed on Feb. 14, 2011 and entitled“Thermal Management for Solid State High-Power Electronics,” the entirecontents of all of which are hereby expressly incorporated by referenceThis patent application is a continuation-in-part patent application of:U.S. Ser. No. 12/290,195 filed on Oct. 28, 2008 and entitled HEATTRANSFER DEVICE; U.S. Ser. No. 12/584,490 filed on Sep. 5, 2009 andentitled HEAT TRANSFER DEVICE; and U.S. Ser. No. 12/932,585 filed onFeb. 28, 2011 and entitled THERMAL INTERFACE DEVICE; the entire contentsof all of which are hereby expressly incorporated by reference.

FIELD OF THE INVENTION

This invention relates generally to a removal of heat fromheat-generating component and more specifically to a removal of heat athigh flux.

BACKGROUND OF THE INVENTION

The subject invention is an apparatus and method for removal of wasteheat from heat-generating components including analog solid-stateelectronics, digital solid-state electronics, semiconductor laserdiodes, light emitting diodes, photo-voltaic cells, vacuum electronics,and solid-state laser crystals.

There are many devices generating waste heat as a byproduct of theirnormal operations. These include analog solid-state electroniccomponents, digital solid-state electronic components, semiconductorlaser diodes, light emitting diodes for solid-state lighting,solid-state laser components, laser crystals, vacuum electroniccomponents, and photovoltaic cells. Waste heat must be efficientlyremoved from such components to prevent overheating and consequentialloss of efficiency, malfunction, or even catastrophic failure. Methodsfor waste heat management may include conductive heat transfer,convective heat transfer, and radiative heat transfer, or variouscombinations thereof. For example, waste heat removed from heatgenerating components may be transferred to a heat sink by a flowingheat transfer fluid. Such a heat transfer fluid is also known as acoolant.

Cooling requirements for the new generation of heat-generatingcomponents (HGC) are very challenging for thermal managementtechnologies of prior art. For example, an ongoing miniaturization ofsemiconductor digital and analog electronic devices requires removal ofheat at ever increasing fluxes now on the order of several hundreds ofwatts per square centimeter. Traditional heat sinks and heat spreadershave large thermal resistance contributing to elevated junctiontemperatures and thus reducing device reliability. As a result, removalof heat often becomes the limiting factor and a barrier to furtherperformance enhancements. More specifically, a new generation ofhigh-power semiconductors for hybrid electric vehicles and futureplug-in hybrid electric vehicles requires improved thermal management toboost heat transfer rates, eliminate hot spots, and reduce volume, whileallowing for higher current density.

High-brightness light emitting diodes (LED) being developed forsolid-state lighting for general illumination in commercial andhousehold applications also require improved thermal management. Thesenew light sources are becoming of increased importance as they offer upto 75% savings in electric power consumption over conventional lightingsystems. Waste heat must be effectively removed from the LED chip toreduce junction temperature, thereby prolonging LED life and making LEDcost effective over traditional lighting sources.

Another class of electronic components requiring improved cooling aresemiconductor-based high-power laser diodes used for direct materialprocessing and pumping of solid-state lasers. Generation of opticaloutput from laser diodes is accompanied by production of large amount ofwaste heat that must be removed at a flux on the order of severalhundreds of watts per square centimeter. In addition, the temperature ofhigh-power laser diodes must be controlled within a narrow range toavoid undesirable shifts in output wavelength.

Photovoltaic cells (solar electric cells and thermo-photovoltaic cells)are becoming increasingly important for generation of electricity. Suchcells may be used with concentrators to increase power generation perunit area of the cell and thus reduce initial installation cost. Thisapproach requires removal of waste heat at increased flux. Similarly,high-performance crystals used in solid-state lasers generate waste heatthat may require removal at fluxes in the neighborhood of thousand wattsper square centimeter.

Anodes in x-ray tubes are subjected to very high thermal loading.Rotating anodes are frequently used to spread the heat to avoidoverheating. Such rotating anodes inside a vacuum enclosure areimpractical for use in a new generation of x-ray tubes for use incompact and portable devices in medical and security applications. Acompact and lightweight heat transfer component having no moving partsinside the vacuum is very desirable.

Current approaches for removal of waste heat at high fluxes include 1)spreading of heat with elements having high thermal conductivity and/or2) forced convection cooling using liquid coolants. However, even withheat spreading materials having extremely high thermal conductivity suchas diamond films and certain graphite fibers, a significant thermalgradient is required to conduct large amount of heat even over shortdistances. In addition, passive heat spreaders are not conducive totemperature control of the HGC. Forced convection methods for removal ofwaste heat at high fluxes may use microchannel heat exchangers orimpingement jets operating at high flow rates to obtain desirable heattransfer coefficient with conventional coolants such as water, alcohol,or ethylene glycol. This results in a very high coolant consumption andrequires a large pumping system. Known forced convection systems havemany components, are bulky, heavy, and have geometries that require thecoolant to make complex directional changes while traversing the coolantloop. Such directional changes are a potential source of increased flowturbulence causing higher pressure drop in the loop and, therefore,necessitate higher pumping power.

Metals have a thermal conductivity several orders-of-magnitude greaterthan water and organic liquids. Liquid (molten) metals have a viscositycomparable to that of water. As a result, liquid metals are excellentcandidates for advantageous cooling in many demanding applications,especially where heat must be removed at high heat flux. Initially,liquid metal cooling was developed for thermal management of nuclearreactors on submarines in the 1950's. These large systems used eutecticalloy of sodium and potassium (also known as NaK) and in some cases,eutectic alloys of lead and bismuth. A large number of patents have beenawarded in connection with these large-scale systems.

Liquid metal cooling for small commercial applications (e.g.,electronics) is deemed to have been enabled by the discovery of a lowmelting point (−19° C.) eutectic alloy of gallium, indium, and tin(galinstan) (see, for example, U.S. Pat. No. 5,800,060). Galinstan isnon-toxic, stable in air, and it wets well many materials. Thisopportunity was recognized in several recent disclosures, for example,U.S. Pat. Nos. 7,505,272, 7,697,291, 7,539,016, 7,764,499, 7,701,716,7,672,129, 7,245,495, 7,861,769, and 7,131,286. To date, no devicesbased on these disclosures are known to have appeared on the market.

The above disclosures typically suggest a traditional layout for athermal management system found already in the above mentioned nuclearsystems: a heat exchanger (HEX) for receiving heat, HEX for rejectingheat, plumbing, and a pump. Such configurations may not self-containedand may be impractical for many applications because they may have alarge size, may not sealed, may use incompatible materials, and may havelarge electromagnetic interference (EMI). In addition, above disclosuresdo not address the challenges of handling and pumping liquid metal,namely:

-   -   1) Galinstan has a specific gravity of about 6.4, which means        that galinstan flow loop may require nearly 7-times more pumping        power to operate than a comparable water flow loop having the        same flow velocity.    -   2) Gallium alloys have a tendency to form amalgams with other        metals, which may result in severe corrosion in commonly used        engineering metals. In addition, the solid inter-metallic        compounds produced by the corrosive action may form deposits        inside the liquid metal flow channel, impeding the heat        transfer, and possibly block the flow channels.    -   3) Pumping of liquid metal with an electromagnetic pump may be        very simple in theory, but it may be challenging in practice due        to possible complex magneto-hydro-dynamic (MHD) boundary layers        and MHD instabilities.    -   4) Volumetric specific heat of liquid metal may be only about        half of that of water. Hence, a liquid metal cooling loop may        require higher flow velocities to carry away the same amount of        heat as a comparable water loop with the same temperature rise.        This means that, a liquid metal cooling loop operating at low        velocity may not be much more effective (and may be actually        less effective) than a comparable water loop.

The above indicates that for a superior performance, a liquid metalcooling hardware may not have an arbitrary configuration and/orarbitrary operating parameters.

In summary, prior art does not teach a heat transfer device capable ofremoving heat at very loads and high fluxes that is also compact,lightweight, self contained, capable of accurate temperature control,has a low thermal resistance, and requires very little power to operate.It is against this background that the significant improvements andadvancements of the present invention have taken place.

SUMMARY OF THE INVENTION

The present invention provides a heat transfer device (HTD) wherein acoolant flows in a closed channel with a substantially constant radiusof curvature. This arrangement offers low resistance to flow, whichallows to flow the coolant at very high velocities and thus enables heattransfer at very high rate while requiring relatively low power tooperate. HTD of the subject invention may be used to cool HGC requiringremoval of waste heat at very high heat flux. Such HGC may includesolid-state electronic chips, semiconductor laser diodes, light emittingdiodes for solid-state lighting, solid-state laser components, lasercrystals, optical components, vacuum electronic components, andphotovoltaic cells. Heat removed by HTD from HGC may be transferred to aheat sink or environment at a reduced heat flux. For example, HTD maytransfer acquired heat to a structure, heat pipe, secondary liquidcoolant, phase change material (PCM), gaseous coolant, or ambient air.

In one preferred embodiment of the present invention, the HTD comprisesa body having a first surface, a second surface, and a closed flowchannel. The first surface is adapted for receiving heat from a heatgenerating component and the second surface is adapted for transferringheat to a heat sink. The flow channel has a substantially constantradius of curvature in the flow direction. An electrically conductiveliquid coolant is flowed inside the flow channel by means of amagneto-hydrodynamic (MHD) effect (MHD drive).

In another preferred embodiment of the present invention, electricallyconductive liquid or ferrofluid coolant may be used and flowed by themeans of a moving magnetic field. Moving magnetic field induces eddycurrents in the electrically conductive coolant that, in turn, provideforce coupling to the coolant (inductive drive). Alternatively, movingmagnetic field directly couples into the ferrofluid (magnetic drive).Suitable moving magnetic field may be generated by a rotating magnet.

In yet another preferred embodiment of the present invention, the moving(rotating or traveling magnetic) magnetic field may be generated bystationary electromagnets operated by alternate current in anappropriate poly-phase relationship. In a still another embodiment ofthe present invention, the coolant is an arbitrary liquid flowed in aclosed channel with a substantially constant radius of curvature. Thecoolant flow is induced by a rotating impeller (impeller drive) spun bya flow of secondary coolant, mechanical means, moving magnetic field, orby electromagnetic induction.

Accordingly, it is an object of the present invention to provide a heattransfer device (HTD) for removing waste heat from HGC. The HTD of thepresent invention is simple, compact, lightweight, self-contained, canbe made of materials with a coefficient of thermal expansion (CTE)matched to that of the HGC, requires relatively little power to operate,and it is suitable for large volume production.

It is another object of the invention to provide means for cooling HGC.

It is still another object of the invention to provide means fortemperature control of HGC.

It is yet another object of the invention to cool a semiconductorelectronic components.

It is yet further object of the invention to cool semiconductor laserdiodes.

It is a further object of the invention to cool LED for solid-statelighting.

It is still further object of the invention to cool computer chips.

It is an additional object of the invention to cool photovoltaic cells.

These and other objects of the present invention will become apparentupon a reading of the following specification and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side cross-sectional view of a heat transfer device (HTD)in accordance with one embodiment of the subject invention using amagneto-hydrodynamic drive.

FIG. 1B is a cross-sectional view of an HTD of FIG. 1A in a planetransverse to coolant flow.

FIG. 2A is an enlarged view of portion 2A of the HTD of FIG. 1A.

FIG. 2B is an enlarged view of portion 2B of the HTD of FIG. 1B.

FIG. 3 is an enlarged view of alternative portion 2B of the HTD of FIG.1B showing a flow channels with surface extensions.

FIG. 4 is an enlarged view of another alternative portion 2B of the HTDof FIG. 1B showing multiple flow channels arranges side-by-side.

FIG. 5 is an enlarged view of portion 2A of the HTD of FIG. 1A showing amounting of a laser diode array HGC.

FIG. 6 is an enlarged view of portion 2A of the HTD of FIG. 1A showing amounting of a laser diode bar HGC.

FIG. 7 is an enlarged view of portion 2A of the HTD of FIG. 1A showing amounting of a light emitting diode HGC.

FIG. 8 is an enlarged view of portion 2A of the HTD of FIG. 1A showing amounting of a solid-state laser crystal HGC.

FIG. 9A shows an alternative HTD body having internal passages for asecondary coolant.

FIG. 9B shows a variant of an alternative HTD having an internal passagefor flowing a secondary coolant.

FIG. 10 shows another alternative HTD body having external fins for heattransfer to gaseous coolant or ambient air.

FIG. 11A is a cross-sectional view of an HTD in accordance with anotherembodiment of the subject invention wherein coolant flow is induced by arotating magnetic field produced by a rotating magnet.

FIG. 11B is a side cross-sectional view of the HTD of FIG. 11A in aplane transverse to coolant flow.

FIG. 12A is a cross-sectional view of an HTD in accordance with a yetanother embodiment of the subject invention wherein coolant flow isinduced by a rotating magnetic field produced by stationaryelectromagnets.

FIG. 12B is a side cross-sectional view of the HTD of FIG. 12A in aplane transverse to the flow loop.

FIG. 12C is a side cross-sectional view of an alternative HTD havingstationary electromagnets with a return flux yoke

FIG. 13 shows a suitable connection of electromagnets to a single phasealternating current supply.

FIG. 14 shows a variant to the HTD in accordance with a yet anotherembodiment of the subject invention wherein the electromagnets arearranged to generate translating magnetic field.

FIG. 15A is a side cross-sectional view of an HTD in accordance withstill another embodiment of the subject invention using an impeller.

FIG. 15B is a side cross-sectional view of the HTD shown in FIG. 15A.

FIG. 15C is a side cross-sectional view of an HTD in accordance with avariant of embodiment of the subject invention shown in FIG. 15A.

FIG. 15D is a side cross-sectional view of the HTD shown in FIG. 15C.

FIG. 16 is a side cross-sectional view of an HTD in accordance with afurther embodiment of the subject invention suitable for cooling byimpingement flow.

FIG. 17 is a side view of an HTD in accordance with a yet furtherembodiment of the subject invention suitable for cooling multiple heatgenerating components.

FIG. 18A is a cross-sectional view 18A-18A of the HTD shown in FIG. 17.

FIG. 18B is a cross-sectional view 18B-18B of the HTD shown in FIG. 18A.

FIG. 19 is an isometric view of the HTD of FIG. 17 with portions ofselected outer elements removed to expose the inner elements and showingadditional components.

FIG. 20 is an exploded isometric view of the HTD of FIG. 17 showingadditional components.

FIG. 21 is an isometric view of the HTD of FIG. 19 indicating the flowpaths of secondary coolant.

FIG. 22 is an isometric view with three-quarter cross-section of the HTDof FIG. 19.

FIG. 23 is an isometric view of an inverter assembly with two HTD's ofFIG. 17 installed, in accordance with a still further preferredembodiment of the subject invention.

FIG. 24 is a side cross-sectional view 24-24 of the inverter assembly ofFIG. 23.

FIG. 25 is an exploded isometric view of the inverter assembly of FIG.23.

FIG. 26 is a side view of an HTD in accordance with a variant of the HTDof FIG. 17 having an externally driven impeller.

FIG. 27A is a cross-sectional view 27A-27A of the HTD shown in FIG. 26.

FIG. 27B is a cross-sectional view 27B-27B of the HTD shown in FIG. 27A.

FIG. 28 is a side view of an HTD in accordance with another variant ofthe HTD of FIG. 17 having an impeller driven by a rotating magneticfield.

FIG. 29A is a cross-sectional view 29A-29A of the HTD shown in FIG. 28.

FIG. 29B is a cross-sectional view 29B-29B of the HTD shown in FIG. 29A.

FIG. 30 is a side view of an HTD in accordance with another variant ofthe HTD of FIG. 17 having a magneto-hydrodynamic pump.

FIG. 31A is a cross-sectional view 31A-31A of the HTD shown in FIG. 30.

FIG. 31B is a cross-sectional view 31B-31B of the HTD shown in FIG. 31A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Selected embodiments of the present invention will now be explained withreference to drawings. In the drawings, identical components areprovided with identical reference symbols in one or more of the figures.It will be apparent to those skilled in the art from this disclosurethat the following descriptions of the embodiments of the presentinvention are merely exemplary in nature and are in no way intended tolimit the invention, its application, or uses.

Referring now to FIGS. 1A and 1B, there is shown a heat transfer device(HTD) 100 in accordance with one preferred embodiment of the subjectinvention. HTD 100 comprises a body 102, magnets 128 a and 128 b,electrodes 130 a and 130 b, and electrical conductors 126 a and 126 b.The body 102 further comprises a first surface 106 adapted for receivingheat from a heat generating component (HGC), a second surface 108adapted for rejecting heat to a heat sink, and a flow channel 104. Thebody 102 is preferably made of material having high thermalconductivity. Preferably, such a material may also have a low electricalconductivity or such a material may be dielectric. Suitable materialsfor construction of the body 102 may include silicon, berylia, siliconcarbide, and aluminum nitride. A heat generating component (HGC) 114 maybe also attached to the first surface 106 and arranged to be in a goodthermal communication therewith. HGC 114 may be, but it is not limitedto a solid-state electronic chip, semiconductor laser diode, lightemitting diodes (LED), solid-state laser crystal, optical component,x-ray tube anode, or a photovoltaic cell. If desired, the body 102 maybe made from material having a coefficient of thermal expansion (CTE)matched to the CTE of the HGC 114. In some variants of the subjectinvention the body 102 can be a composite unit made of several suitablyjoined different materials. The second surface 108 is arranged to be ina good thermal communication with a heat sink (not shown). Suitable heatsinks include a structure, heat pipe, secondary liquid coolant, phasechange material (PCM), gaseous coolant, or ambient air. When a fluidused as a heat sink, it may employ natural convection or forcedconvection to remove heat from the second surface 108. The secondsurface 108 may also include surface extensions such as fins or ribs toenhance heat transfer therefrom.

Referring now to FIGS. 2A and 2B, the HGC 114 may be thermally coupledto the first surface 106 with a suitable joining material 120.Preferably, joining material 120 has a good thermal conductivity.Suitable joining materials include solder, thermally conductive paste,epoxy, liquid metals, and adhesive. Alternatively, HGC 114 may bediffusion bonded onto surface 106. As another alternative, the HGC 114may be mechanically attached onto surface 106. The flow channel 104comprises an outer surface 110 and an inner surface 112. Each of thesurfaces 110 and 112 may have a width “W” and they may be separated fromeach other by a distance “H”. Preferably, the surface 110 has a constantradius of curvature “R” and the inner surface 112 has a radius “R minusH” (“R−H”). For example, surfaces 110 and 112 may each be cylindricaland mutually concentric, thereby giving the flow channel 104 a generalshape of a hollow cylinder with an outer radius “R”, and an inner radius“R−H”, and height “W”. More generally, the flow channel may have a shapeof a toroid, which is a geometrical object generated by revolving ageometrical figure around an axis external to that figure. For example,the geometrical figure revolved may be a polygon. In particular, thegeometrical figure may be a rectangle having a width “W” and height “H”.Because the channel forms a closed loop, it may be also referred to inthis disclosure as the “closed flow channel.” Preferred range for thewidth “W” is 0.1 to 20 millimeters, but dimensions outside this rangemay be also practiced. Preferred range for the outer radius of curvature“R” is 5 to 25 millimeters, but dimensions outside this range may bealso practiced. Preferably, the distance “H” is chosen so that thechannel 104 has a hydraulic diameter (=2WH/(W+H)) about five (5)micrometers to three (3) millimeters, and most preferably about ten (10)micrometers to one (1) millimeter. In addition, surfaces 110 and 112should be made very smooth. Preferably, surfaces 110 and 112 arefinished to surface roughness of less than 8 micrometersroot-mean-square value, and most preferably to surface roughness of lessthan 1 micrometer root-mean-square value. Surfaces of the flow channel104 may also have a coating to protect them from corrosion. The firstsurface 106 may be generally tangential to the outer surface 110 andseparated from it by a distance “S” (FIG. 2B). Preferred range for thedistance “S” is 0.1 to 1 millimeter, but dimensions outside this rangemay be also practiced.

The flow channel 104 contains a suitable electrically conductive liquidcoolant 116. Preferably, the flow channel 104 is not entirely filledwith the liquid coolant and at least some void space free of liquidcoolant is provided inside the channel to allow for thermal expansion ofthe coolant. Preferably, the liquid coolant 116 has a good thermalconductivity, low viscosity, and low freezing point. Suitable liquidcoolants 116 include selected liquid metals. For the purposes of thisdisclosure, the term “liquid metal” shall mean suitable metals (andtheir suitable alloys) that are in a liquid (molten) state at theiroperating temperature. Liquid metals have a comparably good thermalconductivity while being also electrically conductive and, in some caseshave a relatively low viscosity. Examples of suitable liquid metalsinclude mercury, gallium, indium, bismuth, tin, lead, potassium, andsodium. Ordinary or eutectic liquid metal alloys may be used. Examplesof suitable liquid eutectic metal alloys include Indalloy 51 andIndalloy 60 (manufactured by Indium Corporation in Utica, N.Y.),galinstan (obtainable from Geratherm Medical AG in Geschwenda, Germany).Galinstan is a nontoxic eutectic alloy of 68.5% by weight of gallium,21.5% by weight of indium and 10% by weight of tin, having a meltingpoint around minus 19 degrees Centigrade. Examples of suitable liquidmetal alloys may be also found in the U.S. Pat. No. 5,800,060 issued toG. Speckbrock et al., on Sep. 1, 1998. It is important that electrodes130 a and 130 b (FIG. 1B), and surfaces of the flow channel 104 are madeof materials compatible with the coolant 116. In particular, it is wellknow that liquid gallium and its alloys severely corrode many metals.Prior art indicates that certain refractory metals such as tantalum andtungsten may be stable in gallium. See, for example, “Effects of Galliumon Materials at Elevated Temperatures,” by W. D. Wilkinson, ArgonneNational Laboratory Report ANL-5027 (August 1953). To protect againstcorrosion, surfaces of the flow channel 104 may be coated with suitableprotective film. Prior art indicates that TiN and certain organiccoatings may be stable in gallium. If a protective coating isadditionally dielectric, the body 102 may be constructed fromelectrically conductive materials. In particular, TiN and diamond-likecoating (DLC) may provide suitable protection to metals such as aluminumand copper from corrosion by gallium. Diamond-like coating may beobtained from Richter Precision in East Petersburg, Pa.

The outer surface 110 may also include extensions 118 to increase thecontact area between the surface 110 and liquid coolant 116 (FIG. 3).Suitable form of surface extension 118 includes fins and ribs.Alternatively, multiple flow channels 104 a-104 e may be employed (FIG.4). In some variants of the subject invention, a portion of the HGC 114may form a portion of the outer surface 110 of the flow channel 104. Insuch variants of the invention, the liquid coolant 116 may directly weta portion of the surface of the HGC 114. FIG. 5 shows a mounting of HGC114′, which is an array of semiconductor laser diodes (or laser diodebars) 150 imbedded in a substrate 148 and producing optical output 152.Suitable array of semiconductor laser diode bars imbedded in a substrateknown as “silver bullet laser diode assembly submodule” and as “goldenbullet laser diode assembly submodule” may be obtained fromNorthrop-Grumman Cutting Edge Optronics in St. Charles, Mo. FIG. 6 showsa mounting of HGC 114″, which is a laser diode bar producing opticaloutput 152. Suitable laser diode bar known as “unmounted laser diodebar” may be obtained from Northrop-Grumman Cutting Edge Optronics in St.Charles, Mo. FIG. 7 shows a mounting of HGC 114′″, which is a high-powerlight emitting diode producing optical output 153. Suitable high-powerlight emitting diode known as “Luxeon® K2” may be obtained from PhilipsLumileds Lighting Company, Sun Valley, Calif. FIG. 8 shows a mounting ofHGC 114 ^(iv), which is a solid-state laser crystal receiving opticalpump radiation 151 and amplifying a laser beam 155. Suitable solid-statelaser crystal may be in the form of a thin disk laser as, for example,described by Kafka et al., in the U.S. Pat. No. 7,003,011.

Referring now again to FIGS. 1A and 1B, the magnets 128 a and 128 b arearranged to generate magnetic field that traverses the flow channel 104in a substantially radial direction in the proximity of electrodes 130 aand 130 b. Double arrow line 160 indicates preferred directions of themagnetic field. Magnets 128 a and 128 b are preferably permanentmagnets, and most preferably rare earth permanent magnets.Alternatively, magnets 128 a and 128 b may be formed as electromagnets.As a yet another alternative, magnets 128 a and 128 b may be poleextensions of a single magnet. A yoke 131 made of soft ferromagneticmaterial (e.g., iron or soft steel) may be provided to carry return fluxbetween magnets 178 a and 178 b. Electrodes 130 a and 130 b are inelectrical contact with the liquid coolant 116 and are arranged so thatelectric current may be passed through the coolant 116 in the regionbetween the magnets 128 a and 128 b, and in a direction generallyorthogonal to magnetic field direction. Electrodes 130 a and 130 b maybe connected to external source of direct electric current via electricconductors 126 a and 126 b respectively. The HTD 100 may further includea magnetic shield (not shown) to prevent adverse effect of magneticfield generated by magnets 128 a and 128 b on HGC 114 and/or nearlycomponents.

In operation, electric current is passed though the liquid coolant 116between electrodes 130 a and 130 b. Because at least a portion of thecoolant 116 is immersed in magnetic field having a vector componentorthogonal to the electric current flowing though the coolant 116, amagneto-hydrodynamic (MHD) effect causes the coolant 116 to flow in thedirection indicated by the arrow 122 in FIG. 1A and the arrows 124 inFIG. 2A. As a result, flow of coolant 116 forms a closed flow loop.Because the closed flow loop has a substantially constant radius ofcurvature and the walls of the flow channel 104 are smooth, the flow ofcoolant 116 encounters relatively little resistance. As a result, veryhigh flow velocities of coolant 116 can be sustained with a relativelysmall amount of motive power. This disclosure may refer to the means forflowing the coolant by MHD effect as an “MHD drive.”

The HGC 114 is operated and its waste heat is allowed to transferthrough the first surface 106 into the body 102 and conducted to theouter surface 110 of the flow channel 104. The second surface 108 ismaintained at a temperature substantially below the temperature of theHGC 114. Liquid coolant 116 flowing at high velocity enables a very highheat transfer coefficient on the surface 110. Heat is transferred fromthe surface 110 into the liquid coolant 116, transported by the coolant116, and deposited into other parts of the body 102. Heat deposited intoother parts of the body 102 is conducted to the second surface 108 andtransported therefrom to a suitable heat sink. Using the above process,HTD 100 removes heat from the HGC 114 and transfers it to a heat sink orenvironment. FIG. 9A shows an HTD body 102′ having a second surface 108′formed as internal passages for flowing secondary liquid or gaseouscoolant. FIG. 9B shows an HTD body 102′″ having a second surface 108′″formed as an internal passages for flowing secondary liquid or gaseouscoolant 185 in a secondary coolant passage 167 placed along the flowchannel 104. The secondary coolant passage 167 may be also formed as aplurality of channels. Such channels may be generally straight andparallel. Alternatively such channels may be serpentine-like.Preferably, the secondary coolant 185 is flowed in the direction(indicated by arrows 179) generally opposite to the direction of theflow inside the flow channel 104 (indicated by arrows 124). FIG. 10shows an HTD body 102″ having a second surface 108″ formed as externalfins for transferring heat to a liquid coolant, gaseous coolant, orambient air.

Temperature of the HGC 114 may be controlled by controlling the flowvelocity of the coolant 116. The latter can be accomplished bycontrolling the current drawn through the coolant 116 via electrodes 130a and 130 b. For example, by drawing more current through the coolant116, the coolant flow velocity may be increased, and the HGC waste heatmay be removed at a lower temperature differential between the HGC andthe heat sink. Conversely, by drawing less current through the coolant116, the coolant velocity may be decreased, and the HGC waste heat maybe removed at a higher temperature differential between the HGC and theheat sink. Thus, by drawing more current through the coolant 116, thetemperature of the HGC 114 may decreased, and by drawing less currentthrough the coolant 116, the temperature of the HGC 114 may beincreased. An automatic closed-loop temperature control of the HGC 114can be realized by sensing HGC temperature (for example, with athermocouple) and using this information to appropriately control thecurrent drawn through the coolant 116. In particular, if the HGC 114 isan LED, its temperature may be inferred from the output light spectrum.A means for sensing the LED light spectrum may be provided for thispurpose. If the HGC 114 is a semiconductor laser diode, its temperaturemay be inferred from the output light center wavelength. A means forsensing the semiconductor laser diode output light center wavelength maybe provided for this purpose. If the HGC 114 has electric currentsflowing therethrough, HGC temperature may be determined from certaincurrent and/or voltages supplied to or flowing through in the HGC. Ifthe coolant used in the HTD 100 is susceptible to freezing (solidifying)due to ambient conditions during inactivity, the HTD may be equippedwith an electric heater to warm the coolant up to at least its meltingpoint. HGC 114 may be also operated to warm up the HTD.

Referring now to FIGS. 11A and 11B, there is shown a heat transferdevice (HTD) 200 in accordance with another preferred embodiment of thesubject invention. HTD 200 is similar to HTD 100, except that in HTD 200the coolant 216 inside the flow channel 204 may be an electricallyconductive liquid or a ferrofluid. In addition, the flow of the coolant216 is caused by a rotating magnetic field. The flow channel 204 in HTD200 may be of the same construction as the flow channel 104 in HTD 100.Ferrofluids are composed of nanoscale ferromagnetic particles suspendedin a carrier fluid, which may be water, an organic liquid, or othersuitable liquid. Certain water-based ferrofluids such as W11 availablefrom FerroTec in Bedford, N.H., are also electrically conductive.Ferrofluids using a liquid metal or liquid metal alloy as a carrierfluid have been reported in prior art; see, for example, an article byJ. Popplewell and S. Charles in New Sci. 1980, 97(1220), 332. Thenano-particles are usually magnetite, hematite or some other compoundcontaining iron, and are typically on the order of about 10 nanometersin size. This is small enough for thermal agitation to disperse themevenly within a carrier fluid, and for them to contribute to the overallmagnetic response of the fluid. The ferromagnetic nano-particles arecoated with a surfactant to prevent their agglomeration (due to van derWaals and magnetic forces). Ferrofluids may display paramagnetism, andare often referred as being “superparamagnetic” due to their largemagnetic susceptibility. It should be noted that ferrofluid may becomemagnetically saturated at a rather low magnetic fields of less than 0.1Tesla (1,000 gauss). Alternatively, liquid coolant 216 may comprise aliquid having significant paramagnetic, diamagnetic, or ferromagneticproperties.

The body 202 is similar to body 102 of HTD 100 (FIG. 1A) except that ithas a round central opening 264. In addition, the magnets 128 a and 128b, the electrodes 130 a and 130 b, and the electric conductors 126 a and126 b (FIG. 1A) are omitted. The body 202 further comprises a firstsurface 206 adapted for receiving heat from HGC 114, a second surface208 adapted for transferring heat to a suitable heat sink. Furthermore,the body 202 may be also constructed from a variety of materialspreferably having high thermal conductivity. For example, the body 202may be constructed from copper, copper-tungsten alloy, aluminum,molybdenum, silicon, and silicon carbide. The body 202 may also beconstructed in-part or in-whole from ferromagnetic materials to providereturn for magnetic flux lines and/or to shield adjacent components frommagnetic field. Depending on the choice of coolant 216, the surfaces ofthe flow channel 204 may require appropriate protective coating toprevent corrosion. HTD 200 further comprises a magnet 234 rotatablysuspended inside the opening 264 and positioned so that a significantportion of magnetic field lines cross the flow channel 204. The label“N” designates the north pole of the magnet and the label “S” designatesthe south pole of the magnet 234. The magnet 234 and the ferromagneticmaterial in the body 202 (if used) are preferably arranged so that whenthe magnet 234 is rotated, a given portion of the coolant 216 isalternatively exposed to large variations in magnetic field level, andmost preferably to a magnetic field with alternating direction. When thecoolant 216 is a ferrofluid, the variations in magnetic field amplitudeshould include magnetic field level substantially lower than itssaturation magnetic field. Preferably, the magnetic field within saidcoolant may include magnetic field values of less than 50 Gauss (0.005Tesla).

Operation of HTD 200 is similar to the operation of HTD 100 except thatthe flow of the coolant 216 is caused by different means than flow ofthe coolant 116 in HTD 100. In particular, magnet 234 is rotated in thedirection of arrow 238 to generate a rotating magnetic field. The magnet234 may be rotated mechanically by a shaft 236 that may be coupled to anexternal drive such as an electric motor. For example, if the surface208 is cooled by air (see, e.g., FIG. 10) supplied by a fan driven by anelectric motor, the magnet 234 may be attached to the output shaft ofthat motor. Alternatively, the magnet 234 may be rotated by means of amagnetic coupling to an external rotating magnetic component. As anotheralternative, the magnet 234 may be rotated by a rotating magnetic fieldgenerated by electromagnets. As a yet another alternative, the magnet234 may be rotated by a turbine operated by a secondary coolant flowingthrough the central opening 264.

If the coolant 216 is an electrically conductive liquid, time varyingmagnetic field produced by the rotation of the magnet 234 induces eddycurrents in the electrically conductive coolant 216. Such eddy currents,interact with the rotating magnetic field produced by the magnet 234thereby establishing a force coupling between the rotating magnet 234and the coolant 216. As a result, rotating magnet 234 exerts a forceonto the coolant 216 causing the coolant 216 to flow inside the flowchannel 204 in the direction of the arrow 222 thereby forming a flowloop. This disclosure may refer to the means for flowing an electricallyconductive coolant by rotating magnetic field as an “inductive drive.”Additional information about eddy current devices may be found in“Permanent Magnets in Theory and Practice,” chapter 7.6: Eddy-CurrentDevices, by Malcolm McCraig, published by Pentech Press, Plymouth, UK,1977; and in “An Introduction to Magnetohydrodynamics,” chapter 5,section 5.5: Rotating Fields and Swirling Motions, by P.A. Davidson,published by Cambridge Texts in Applied Mathematics, CambridgeUniversity Press, Cambridge, UK, 2001.

If the coolant 216 is a ferrofluid, magnetic field produced by therotating magnet 234 directly couples into the coolant 216 and flows itinside the flow channel 104 in the direction of the arrow 222.Rotational speed of the magnet 234 may used to control the flow velocityof the coolant 216. Thus, controlling the rotational speed of the magnet234 allows to control the rate of heat removal from the HGC 114 and,thereby to control the HGC temperature. This disclosure may refer to themeans for flowing ferrofluid coolant by rotating magnetic field as“magnetic drive.”

Referring now to FIGS. 12A and 12B, there is shown a heat transferdevice (HTD) 300 in accordance with yet another preferred embodiment ofthe subject invention. HTD 300 is essentially the same as HTD 200,except that in HTD 300 the rotating magnetic field for flowing theliquid coolant 216 is generated by stationary electromagnet coils 332 a,332 b, and 332 c, rather than a rotating magnet 234. The coils 332 a,332 b, and 332 are preferably installed inside the central opening 264as shown in FIG. 4A, and supplied with poly-phase alternating electriccurrents. Phases of the alternating currents supplied to the coils 332a, 332 b, and 332 c are set so that the combined magnetic field producedby the coils has a rotating component. For example, the electromagnetcoils 332 a, 332 b, and 332 c may be connected in a delta or star (Y)configuration as is often practiced in the art of three-phasealternating current systems (see, for example, “Standard Handbook forElectrical Engineers,” D. G. Fink, editor-in-chief, Section 2: Electricand Magnetic Systems, Three-Phase Systems, Tenth Edition, published byMcGraw-Hill Book Company, New York, N.Y., 1968) and supplied with anordinary three-phase alternating current. Rotating magnetic fieldcouples into the coolant in an already described manner and causes thecoolant 216 to flow around the closed loop.

One skilled in the art can appreciate that there is a variety ofelectromagnet coil configurations fed by poly-phase alternating currentsthat can produce a time varying magnetic field with a rotating component(see, for example, “Magnetoelectric Devices, Transducers, Transformers,and Machines,” by Gordon D. Slemon, Chapter 5: Polyphase Machines,published by John Willey & Sons, New York, N.Y., 1966). Electromagnetcoils may have ferromagnetic cores such as practiced on electric motorsfor alternating current. FIG. 12C shows a heat transfer device (HTD)300″ which is variant of the HTD 300. The HTD 300″ has electromagnetcoils 332 a, 332 b, and 332 c mounted on a flux return 333 locatedoutside the HTD body 202′. The flux return 333 is preferably made ofsuitable soft ferromagnetic material. The body 202′ is preferably madefrom non-magnetic material except for the core portion 203, which ispreferably made from a suitable soft ferromagnetic material. Suitablesoft ferromagnetic material may include iron, silicon steel, or vanadiumpermendur. Preferably, suitable soft ferromagnetic material may beprovided in form of thin sheets.

If only a single phase current is available, electromagnet coils 332 a,332 b, and 332 c may be combined with a capacitor 356 as shown, forexample, in FIG. 13 to produce a suitable rotating magnetic field. Thereis a variety of similar connections practiced in the art of single phaseelectric motors. Frequency of the alternating currents supplied to theelectromagnet coils 332 a, 332 b, and 332 c may be used to control theflow velocity of the coolant 216. Thus, controlling the frequency of thealternating currents allows to control of the rate for heat removal fromthe HGC 114 and the HGC temperature. Typical range for alternatingcurrent frequency is from 1 to 1000 cycles per second. Alternatively,the coolant flow velocity may be controlled by controlling the electriccurrent supplied to the electromagnets.

FIG. 14 shows an HTD 300′ that is a variant to the HTD 300 wherein theelectromagnet coils 332 a, 332 b, and 332 c are arranged to generate atraveling magnetic field rather than a rotating magnetic field. Inparticular, the electromagnet coils 332 a, 332 b, and 332 c are arrangedas often practiced in the art of linear electric motors and suppliedwith poly-phase alternating current in appropriate phase relationship.The resulting magnetic field is traveling generally in a linear path andit couples into the electrically conductive or ferrofluid coolant in themanner already described in connection with the HTD 300. It can beappreciated by those skilled in the art that the traveling magneticfield may cause the coolant 216 to flow even if the flow channel 204 maynot have a substantially constant radius of curvature.

Referring now to FIGS. 15A and 15B, there is shown a heat transferdevice (HTD) 400 in accordance with still another preferred embodimentof the subject invention. HTD 400 is similar to HTD 100, except that inHTD 400 the flow channel 404 is formed by a gap between the outersurface 410 of body 402 and a cylindrical surface 444 of an impeller440. The impeller 440, which may have a shape of a cylinder is arotatably suspended on bearings 442 and it may be magnetically orinductively coupled to external actuation means. Alternatively, theimpeller may be driven by mechanical means. The body 402 furthercomprises a first surface 406 adapted for receiving heat from a heatgenerating component (HGC), a second surface 408 adapted for rejectingheat. The flow channel 404 contains a liquid coolant 416. The coolant416 preferably has a good thermal conductivity and low viscosity. Forexample, coolant 416 may be substantially water, or alcohol, or mixtureof water and alcohol, aqueous solution of ethylene glycol, or arefrigerant such as Freon.

The coolant 416 may also comprise a fluid containing nanometer-sizedparticles (nanoparticles) also known as nanofluid. Nanofluids areengineered colloidal suspensions of nanoparticles in a base fluid. Thenanoparticles used in nanofluids may be typically made of metals,oxides, carbides, or carbon nanotubes. Common base fluids may includewater and ethylene glycol. Nanofluids may exhibit enhanced thermalconductivity and the convective heat transfer coefficient compared tothe base fluid.

In operation, external actuation means may be used to spin the impeller440. Due to its finite viscosity, at least a portion of the coolant 416is entrained by the cylindrical surface 444 and travels with it, therebyestablishing a flow loop. If desired, the cylindrical surface 444 mayhave surface extensions (for example, ridges, grooves, or surfaceirregularities) to better entrain the coolant. Rotational speed of theimpeller 440 may be used to control the velocity of the coolant 416.Thus, controlling the rotational speed of the impeller 440 allows tocontrol the HGC temperature. This disclosure may refer to the means forflowing a coolant by a rotating impeller as an “impeller drive.”

Referring now to FIGS. 15C and 15D, there is shown a heat transferdevice (HTD) 400′ which is a variant of the HTD 400. The HTD 400′ issimilar to HTD 400, except that the HTD 400′ additionally comprises aninlet port 405 and an outlet port 407 installed in the body 402′. Theinlet port 405 allows for the liquid coolant 416 to be fed from anoutside source into the flow channel 404′, The outlet port 407 allowsfor the liquid coolant 416 to be drained from the flow channel 404′ tothe exterior of the HTD 400′. Furthermore, the second surface 408′ maynot be relied on for rejecting heat.

In operation, liquid coolant 416 is fed from an external supply throughthe inlet port 405 into the flow channel 404′, it is caused to flowunder the HGC 114, and it is drained out of the flow channel 404′through the outlet port 407. External actuation means are provided tospin the impeller 440. Due to the finite viscosity of the coolant 416,at least a portion of the coolant 416 may be entrained by thecylindrical surface 444 of the impeller 440 and may travel with it. As aresult, the flow of coolant 416 from the inlet port 405 to the outletport 407 may be significantly enhanced. Preferably, the cylindricalsurface 444 may have surface extensions (for example, ridges, grooves,or surface irregularities) to better entrain the coolant. Furthermore,the high rotational speed of the impeller 440 may cause and/or enhancethe turbulence in the liquid coolant 416. As a consequence, heattransfer from HGC 114 to the coolant 416 may be significantly enhanced.In particular, for a given flow rate of liquid coolant 416 into theinlet port 405, action of the rotating impeller 440 may significantlyenhance heat transfer over what may be achievable with a stationaryimpeller. The heat acquired by the liquid coolant 416 from the HGC 114is removed from the HTD 400′ by the flow of liquid coolant 416 drainedthrough the outlet port 407. Controlling the rotational speed of theimpeller 440 allows one to control the temperature of HGC 114. Ifdesired, a portion 404 a of the flow channel 404 may be narrowed down toreduce a flow of liquid coolant 416 therethrough.

One important application of the HTD 400′ may be in coolingsemiconductor chips in electronic inverters used in hybrid electricvehicles. In particular, the liquid coolant 416 may be an engine coolantsupplied by the engine cooling loop.

Referring now to FIG. 16, there is shown an HTD 500 in accordance with afurther preferred embodiment of the subject invention comprising a body502 having a flow channel 504 with a flow diverter 565. The flow channel504 has a generally constant radius of curvature except for the flowchannel portion in proximity of the HGC 114. In particular, in proximityof the HGC 114 the flow channel 504 includes the flow director 565arranged to redirect the flow of the coolant 516 indicated by arrows 524in a generally radial direction and to impinge into the channel walljust under the HGC 114. As a result, the heat transfer just under theHGC 114 may be substantially enhanced.

Referring now to FIGS. 17, 18A, 18B, and 19 through 22, there is shownan HTD 600 in accordance with a yet further preferred embodiment of thesubject invention. The HTD 600 comprises a body 602, jacket 689, bushing657, 3-phase armature 683, and 3-phase coils 632 a, 632 b, and 632 c.The body 602 is generally formed as a hollow cylinder having a smoothcircular bore and an exterior surface. The body 602 is preferably madeof material having high thermal conductivity, such as, but not limitedto copper, aluminum, molybdenum, copper-tungsten alloy, silicon carbide(SiC), silicon (Si), aluminum nitride (AlN), beryllium oxide (BeO),boron nitride, as well as metal matrix composites comprising of graphiteor graphine. If the body 602 is made of copper or aluminum alloys, itssurfaces in contact with the liquid metal 616 should be protected with asuitable anticorrosion coating such as, but not limited to, titaniumnitride (TiN) coating or diamond like coating (DLC). The exteriorsurface of the body 602 may also have three flat surfaces 606 formounting HGC 614 and for receiving heat. The exterior surface of thebody 602 may be equipped with channels 667 for flowing a secondarycoolant indicated by arrow(s) 679. A family of suitable secondarycoolants may include, but is not limited to, water, ethylene glycol,alcohol, antifreeze, Freon, air, or other suitable fluids.

The jacket 689 is generally formed as a hollow cylinder with innerdiameter sized to closely fit over the exterior surface of the body 602.The jacket 689 may be made of soft ferromagnetic material. To limitlosses due hysteresis, the jacket is preferably made of silicon steel.For improved resistance to corrosion by the secondary coolant 679, thejacket 689 may be made of ferritic stainless steel, such as the AmericanIron and Steel Institute (AISI) grades 405, 429, 430, 434, 436, and 446.The jacket 689 has three openings 671 (FIG. 20) designed to align withthe flat surfaces 606 on the body 602 when the jacket 689 is installedover the body. In addition, the jacket 689 may have a groove 619. Thejacket 689 may be installed over the body 602 and affixed to it usingbrazing, soldering, swaging, adhesive bonding, or any other suitablejoining techniques. When the jacket 689 is installed over the body 602,the groove 619 connects the secondary coolant flow channels 667 of theexterior of the body 602. Alternatively, a circumferential grooveconnecting the channels 667 may be provided directly on the externalsurface of the body 602.

When the jacket 689 is installed over the body 602, one or more HGC 614may be affixed onto the surface 606 of the body 602. The HGC 614 may bea semiconductor chip die with a Si, SiC, or other suitable substrate.Alternatively, the HGC 614 may be semiconductor chip packaged insuitable casing. For example, each surface 606 may receive two HGC 614,one being an insulated gate bipolar transistor (IGBT) and the other adiode, such as may be used in switching high electric currents. Inparticular, such IGBT-diode combination may be used to constructelectronic inverters for producing 3-phase output from a DC input.

The HGC 614 may be affixed to the surface 606 by soldering, adhesivebonding, or other suitable joining technique. If the body 602 is made ofSiC, AlN, BeO or alike, the surface 606 may be equipped with suitablemetallic coating to facilitate soldering. If the body 602 is made ofelectrically conductive material and electrical insulation between HGC614 and the body 602 is required, a thin (for example, 100-micron thick)wafer 693 of suitable electrically insulating material (for example,AlN) may be placed between the HGC 614 and the surface 606. The diameterof the body 602 is preferably made 4 to 10 times the cross-sectionalwidth of the HGC 614 in FIG. 18A.

The bushing 657 may be generally formed as a hollow cylinder with anoutside diameter to fit the bore of the body 602 and an inside diameterto fit over the 3-phase armature 683. The bushing 657 has groove 673.Preferably the groove 673 has a rectangular shape and it is wide andshallow. The groove 673 may be at least as wide as the HGC 614. Forexample, in some variants of the subject invention, the groove 673 maybe 12 millimeters wide and 1 millimeter deep. The bushing 657 ispreferably made of electrically insulating material such as plastic,glass-filled epoxy, glass ceramic (such as Macor), or ceramic. However,the bushing 657 may be also made from metal. When necessary, portions ofthe metal bushing 657 should be protected with a suitable anticorrosioncoating to avoid corrosion by liquid metal 616. The bushing 657 may beinstalled and affixed into the bore of the body 602 by using adhesives,or by press fitting, or the combination of both, or by any othersuitable technique. When the bushing 657 is installed in the bore of thebody 602, the grove 673 and a portion of the bore form a flow channel604.

The flow channel 604 may be either partially or entirely filled with asuitable liquid metal 616. The liquid metal 616 may be injected into theflow channel by a hypodermic needle via a small delivery hole 607 in thebushing 657. A grove in the cylindrical surface of the bushing 657 maybe used instead of the hole 607. After the flow channel is filled to adesirable level, the delivery hole 607 may be plugged with suitablematerial. For example, the hole may be plugged with suitable adhesive.

The 3-phase armature 683 is arranged to receive the 3-phase coils 632 a,632 b, and 632 c. The coils may be electrically joined in a standarddelta connection and connected to a 3-phase power supply. The armature683 is preferably made from soft ferromagnetic material having lowhysteresis, such as silicon steel. Most preferably, the armature is madefrom silicon steel sheets (also known as transformer plates). Thisapproach reduces eddy current loss. The HTD 600 may also include an endcap 691 and a mounting screw 687 (FIGS. 19-22).

In operation, the 3-phase coils 632 a, 632 b, and 632 c may be energizedwith a 3-phase alternating current (AC) to produce electromagnetic field(EM) in the armature 683, the jacket 689, and the gap therebetween. TheEM field may have a rotating component. The liquid metal 616 may be inthe flow channel 604 located the gap between the armature 683 and thejacket 689, and it may be exposed to the EM field. Because the liquidmetal 616 is electrically conductive, the EM field may generate eddycurrents therein, thus establishing a force coupling between the liquidmetal 616 and the EM field. As a result, the liquid metal 616 may bemade to flow in the channel 604 (FIGS. 18A and 18B) in the directionindicated by arrows 624 (FIG. 18A). Concurrently, a secondary coolantstreams 679 may be injected into channels 667 a and 667 b (FIGS. 18B and21) and flow up to the groove 619 where they may merge, follow thegroove 619 to the channels 679 c and exit as a coolant stream 679′.

The HGC 614 may be operated as intended, thus producing waste heat,which is conducted through the joining material 620, the electricalinsulator 693 (if used), and surface 606 into the body 602. The heat maybe then transported from the portion of body 602 adjacent to the HGC 614into the liquid metal 616 and carried away by the flow. The heat may bethen transported from the liquid metal 616 into the portion of the body602 adjacent to the channels 667, and therefrom into the coolant stream679 flowing through the channels. The liquid metal 616 may remove heatat high flux from the portion of body 602 adjacent to the HGC 614 withvery low thermal resistance and carry it into the portion of the body602 adjacent to the channels 667. The combined area of the channels 667wetted by the coolant stream 679 may be arranged to be many times(preferably 10 to 30 times) larger then the area of the HGC 614thermally contacted to the surface 606. This arrangement may make itpossible to transfer heat into the coolant stream 679 with a low thermalresistance. As a result, the HTD 600 may enable removal of high loadheat at high flux from HGC 614 and transfer it to the secondary coolantstream 679 with very low resistance. Because the flow channel 604 has aconstant curvature, the liquid metal 616 can be flowed at high velocity(up to several meters per second) with only modest motive power.

As already noted above, the HGC 614 may comprise IGBT and diode such asmay be used in electronic inverters for producing 3-phase output from aDC input. Such inverters may be used, for example, in hybrid electricvehicles, all-electric vehicles, photovoltaic power plants, and windpower plants. Referring now to FIGS. 23, 24, and 25, there is shown aninverter assembly 601 in accordance with a still further preferredembodiment of the subject invention. The inverter assembly 601 maycomprise two HTD 600 mounted on a coolant manifold 627 (FIGS. 24 and 25)and integrated with an electronic inverter card 661 populated withcontrol chips, bus bars, and terminals. The coolant manifold 627 maycomprise two ports 613 and coolant tubes 673. The ports 613 comprisethree coolant passages 659 and a gasket 645. The HTD 600 may be insertedinto the ports 613 so that the passages 659 of the ports are alignedwith the coolant channels 667 of the HTD for feeding coolant stream 679to and from the HTD 600. The HTD is help in place in the port 613 by thecombination of the mounting screw 687 and the end cap 691. For clarity,FIGS. 23-25 do not show the electrical connections between the invertercard 661 and the HGC 614 on the HTD 600. Spacers 681 a and 681 b may besued to hold the armature 683 in place. Because the inverter in a hybridelectric vehicle handles only transient loads, the HTD 600 drive(excitation of the 3-phase coils 632 a, 632 b, and 632 c) may be onlyactivated on demand, such as when the vehicle is accelerating. In anall-electric vehicle, the HTD 600 may be operated continuously. Atemperature sensor may be provided to on the HTD 600 or on the HGC 614to warn of overheating. An “over-temperature” signal from the sensor maybe used to shut down the inverter or to limit its power throughput.

Referring now to FIGS. 26, 27A, and 27B, there is shown an HTD 700 inaccordance with a variant to the HTD 600 of the subject invention. TheHTD 700 may be very similar to the HTD 600 except that the flow ofliquid metal 716 is facilitated by an impeller 740 rather than EM field.The bushing 657 for HTD 600 (FIGS. 18A and 18B) is omitted. The impeller740 may be formed as a cylinder mounted on a shaft 715 rotatablysuspended in plugs 721 and 723. The plugs 721 and 723 may include lowfriction or antifriction bearings (not shown) to allow for rotation ofthe shaft 715 with only little torque. The plugs 721 and 723 may be madeof plastic (for example, Nylon) or other suitable material and they maybe press-fitted into the bore of the body 702. O-rings 749 may beprovided to ensure good seal. Selected surfaces of the plugs 721 and723, the body 702, and the impeller 740 may form the flow channel 704filled with liquid metal 716. Suitable seal may be provided around theshaft 715 in the plug 723 to prevent the liquid metal 716 from leaking.Alternatively, the HTD 700 may be positioned with the end of the shaft715 protruding through the plug 723 pointing up.

The cylindrical surface of the impeller 740 may be smooth or it may havegrooves or dents to better engage the liquid metal 716 and to mix it.Suitable grooves may be circumferential, axial, crisscross, may form apattern, or be random in size and/or direction. Additional grooves maybe added onto the flat sides of the impeller 740 to bring in liquidmetal 716 and to allow formation of a lubricating film between theimpeller 740 and the plugs 721 and 723. This embodiment of the subjectinvention allows for using alternative liquids to the liquid metal 716.For example, the liquid metal 716 may be substituted with a coolantcomprising substantially water, or alcohol, or mixture of water andalcohol, or Freon, or nanofluid. The impeller 740 may be formed frommetal, plastic, ceramic, glass, or other suitable material. The jacket789 may be formed from plastic, rubber, metal, or ceramic. If the jacket789 is formed from ductile material, it may be press-fitted,shrunk-fitted, or swaged over the body 702. The body 702 may be formedthe same way as the body 602 of FIGS. 17, 18A, 18B, and 19 through 22.

In operation, the shaft 715 is rotated as indicated by the arrow 746.Rotation of the shaft 715 may be accomplished by external means such as,but not limited to electric motor, internal combustion engine, hydraulicmotor, compressed air turbine, and wind turbine. The impeller 740induces the liquid metal (or alternative coolant, if used) to flowinside the channel 704 in the direction indicated by arrows 724 (FIG.27A). As in the HTD 600, waste heat from HGC 714 is transported into theflow of liquid metal 716 (or alternative coolant, if used) and therefromto the secondary coolant stream 779 flowing through the channels 767.

Referring now to FIGS. 28, 29A, and 29B, there is shown an HTD 800 inaccordance with a variant to the HTD 700 of the subject invention. TheHTD 800 may be very similar to the HTD 700 except that the impeller 840is rotated by a 3-phase electromagnet formed by the armature 831 and3-phase coils 832 a, 832 b, and 832 c. The impeller 840 may be mountedon a shaft 815 rotatably suspended in bearings 809 installed in theplugs 821. Alternatively, the shaft 815 may be stationary, and theimpeller 840 may be rotatably mounted on the shaft 815 via a suitablebearing. The bearings 809 may be friction bearings made of suitablelow-friction material such as Nylon or Teflon, or they may be formed asantifriction bearings with suitable rolling elements, or they may beformed as jewel bearings such as used in precision instruments. Thisembodiment of the subject invention allows for using alternative liquidsto the liquid metal 816. For example, the liquid metal 816 may besubstituted with a coolant comprising substantially water, or alcohol,or mixture of water and alcohol, or Freon, or nanofluid. The impeller840 may be formed similarly to the impeller 740 but it may also comprisea squirrel cage electrical conductor such as practices in rotors ofcertain 3-phase motors. Suitable armature formed from a ferromagneticmaterial may be also included the impeller 840. Alternatively, theimpeller 840 may comprise a permanent magnet having a magnetizationsubstantially in a radial direction of the impeller 840. The armature831 may be formed from a soft ferromagnetic material having lowhysteresis such as silicon steel. Preferably, the armature 831 may beformed from transformer plates.

The HTD 800 may operate in the same manner as the HTD 700, except thatthe motive power to the impeller 840 is provided by the EM fieldgenerated by the 3-phase coils 832 a, 832 b, and 832 c fed by a 3-phaseAC current in concert with the armature 831.

Referring now to FIGS. 30, 31A, and 31B, there is shown an HTD 900 inaccordance with a variant to the HTD 600 of the subject invention. TheHTD 900 may be very similar to the HTD 600 except that the liquid metal916 inside the flow channel 904 is now flowed by magneto-hydro-dynamic(MHD) effect generated by electrodes 930 aa, 930 ab, 930 ba, and 903 bb,and permanent magnet 929. In particular, the permanent magnet 929 has amagnetization indicated by the arrow 943. The permanent magnet 929 ispreferably substantially formed from samarium-cobalt or fromneodymium-iron-boron materials. The bushing 947 may be very similar tothe bushing 657 of the HTD 600, except that it has a provision forinstallation of the electrodes 930 aa, 930 ab, 930 ba, and 903 bb. Thebushing 947 is preferably made of electrically insulating material suchas, but not limited to plastic, Macor®, ceramic, or glass. The jacket989 is preferably made of soft ferromagnetic material preferably havinghigh magnetic saturation, such as, but not limited to, iron, low carbonsteel, vanadium supermendur, or Hiperco®. For improved resistance tocorrosion by the secondary coolant 979, the jacket 989 may be made offerritic stainless steel, such as the American Iron and Steel Institute(AISI) grades 405, 429, 430, 434, 436, and 446.

To prevent corrosion by liquid metal 916, the electrodes 930 aa, 930 ab,930 ba, and 903 bb should be made of corrosion resistant materialpreferably being also a good electrical conductor, such as, but notlimited to molybdenum, tungsten, niobium, or tantalum. Alternatively,the electrode may be made of copper or copper alloy and it may be platedwith a suitable refractory metal such as, but not limited to molybdenum,tungsten, niobium, tantalum, rhenium, osmium, and iridium. The body 902is preferably made of materials having high thermal conductivity and,low electrical conductivity or being dielectric. Suitable materials forthe body 902 may include, but are not limited to silicon carbide,silicon, aluminum nitride, and BeO (beryllia). The electrodes 930 aa,930 ab, 930 ba, and 903 bb may be held in place with suitable adhesivesuch as, but not limited to epoxy or polyacrylate cement. The liquidmetal 719 may be delivered into the channel 904 though the electrodeslots in the bushing 947 prior to installation of the last electrode.

In operation, the electrodes 930 aa and 930 ba (FIG. 31B) may beconnected to a source of direct current so that electric current may beflowed through the portion of liquid metal 916 between the electrodes930 aa and 930 ba. This portion of the liquid metal is also immersed inthe magnetic field generated by the magnet 929. Magnetic field lines areindicated by arrows 925 in FIGS. 31A and 31B. The interaction betweenthe electric current and the magnetic field in the liquid metal 916generates a force on the liquid metal 916 causing it to flow in thedirection indicated by the arrows 924. In some variants of theinvention, the electrodes 930 ab and 930 bb may be omitted. However, ifthe electrodes 930 ab and 930 bb are used, they may be also connected toa source of direct current so that electric current may be flowedthrough the portion of liquid metal 916 between the electrodes 930 aband 930 bb. Care should be exercised to as to the polarity of theelectric current connections to the electrodes 930 aa, 930 ab, 930 ba,and 903 bb to ensure that the MHD forces onto the liquid metal 916 havea consistent direction. In some variant of the invention, the electrodepairs 930 aa-930 ba and 930 ab-903 bb may be electrically connected inseries. For example, the electrodes 930 ba and 930 bb may beelectrically connected with suitable “jumper” conductor to connect theelectrode pairs 930 aa-930 ba and 930 ab-903 bb in series. In this case,the electrodes 930 aa and 930 ab may be connected to a source of directcurrent.

The HTD 900 may operate in the same manner as the HTD 600, except thatthe motive power to the liquid metal 916 is provided by the MHD effectgenerated by electrodes 930 aa, 930 ab, 930 ba, and 903 bb, andpermanent magnet 929 in HTD 900 instead of the EM field generated by thecoils 632 a-c in HTD 600.

An alternative liquid metal alloy disclosed by Brandeburg et al. in theU.S. Pat. No. 7,726,972 and having reportedly extended usefultemperature range may be also usable with the subject invention. TheBrandeburg's alloy differs from the commercially availableGallium-Indium-Tin (GaInSn) alloy in that its composition additionallyincludes 2%-10% Zinc (Zn). A preferred composition of the new alloy,referred to herein as GaInSnZn, contains approximately 3.0% Zn. Like theknown alloy GaInSn, the new alloy GaInSnZn is liquid at ambienttemperatures, but unlike GaInSn, the new alloy GaInSnZn has asubstantially lower melting point. According to Brandeburg et al., thetemperature scan analysis of the new alloy GaInSnZn exhibits a meltingpoint of −36.degree C., and experimental testing has shown that itoperates satisfactorily in the subject apparatus at temperatures as lowas −40.degree C. A further advantage of the new alloy GaInSnZn relativeto the known alloy GaInSn is that the constituent element Zinc isrelatively low in cost compared to the other elements of thecomposition, thereby lowering the cost of the alloy, even as its meltingpoint is significantly lowered.

While the preferred Brandeburg's alloy composition includes 3% Zinc asdescribed in the preceding paragraph, it should be appreciated thatacceptable results for many liquid metal rotary connector applicationsmay be achieved with a GaInSnZn alloy, where Zinc is present in aconcentration range of 2%-10%. Also, alloys additionally containing upto 5% Bismuth (Bi) will provide acceptable results in the subjectapplication. The following table sets forth three potential GaInSnZnalloy compositions, with Zinc present in concentrations of 3%, 5% and7%, along with lower and upper ranges for each of the constituentelements.

Gn In Sn Zn Bi 3% Zn 66.4% 20.9% 9.7% 3.0% 5% Zn 65.1% 20.4% 9.5% 5.0%7% Zn 63.7% 20.0% 9.3% 7.0% Lower   60%   18%   8%   2% 0% Upper   70%  22%  12%  10% 5%

The HTD 700 and HTD 800 of the subject invention may be also practicedwith a liquid coolant suitable for boiling heat transfer in lieu of theliquid metal 716 and 816 respectively. Coolant suitable for boiling heattransfer may include suitable fluorocarbon (Freon) refrigerant, keton(such as acetone), or alcohol (such as ethanol or methanol), or ammonia.The coolant flow channel 704 and 804 respectively may also include avoid that is substantially free of liquid and may contain gases and/orvapors at a predetermined pressure. The void space allows for thermalexpansion of the coolant and for formation of vapor bubbles from liquidcoolant while avoiding excessive buildup of pressure inside the flowchannel.

In operation, when the coolant suitable for boiling heat transferreceives heat, a portion of the high vapor pressure liquid undergoesnucleate boiling. Vapor bubbles are swept by the flow of coolant.Centrifugal force induces hydrostatic pressure within coolant, which maymake the vapor bubbles buoyant. As a result, vapor bubbles may move awayfrom the heat input surface and into the bulk flow of coolant, wherethey may collapse and deposit thermal energy.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” and “includes” and/or “including” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

The terms of degree such as “substantially”, “about” and “approximately”as used herein mean a reasonable amount of deviation of the modifiedterm such that the end result is not significantly changed. For example,these terms can be construed as including a deviation of at least ±5% ofthe modified term if this deviation would not negate the meaning of theword it modifies.

The term “suitable,” as used herein, means having characteristics thatare sufficient to produce a desired result. Suitability for the intendedpurpose can be determined by one of ordinary skill in the art using onlyroutine experimentation.

Moreover, terms that are expressed as “means-plus function” in theclaims should include any structure that can be utilized to carry outthe function of that part of the present invention. In addition, theterm “configured” as used herein to describe a component, section orpart of a device includes hardware and/or software that is constructedand/or programmed to carry out the desired function.

Different aspects of the invention may be combined in any suitable way.

While only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the present invention asdefined in the appended claims. Furthermore, the foregoing descriptionof the embodiments according to the present invention are provided forillustration only, and not for the purpose of limiting the presentinvention as defined by the appended claims and their equivalents. Thus,the scope of the present invention is not limited to the disclosedembodiments.

1. A heat transfer device comprising: a) a body having a first surface,a second surface, and a chamber; said chamber being formed as a hollowcylinder comprising an inner cylindrical surface and an outercylindrical surface; said outer cylindrical surface comprising a centralaxis of symmetry, a first constant radius of curvature, and an azimuthaldirection; said inner cylindrical surface comprising a second constantradius of curvature; said inner cylindrical surface being substantiallyconcentric with said outer cylindrical surface; said first surface isarranged to be in a good thermal communication with a heat generatingcomponent; said first surface being generally tangential to said outercylindrical surface with only a small separation between the two; saidsecond surface is arranged to be to be in a good thermal communicationwith a heat sink; b) a liquid coolant substantially filling saidchamber; and c) a means for flowing said liquid coolant in saidazimuthal direction.
 2. The heat transfer device of claim 1, wherein twotimes the height of said hollow cylinder multiplied by the differencebetween said first constant radius of curvature and said second constantradius of curvature, divided by the sum of said height and saiddifference is in the range of 10 to 2000 micrometers.
 3. The heattransfer device of claim 1, wherein said first constant radius ofcurvature is between 5 and 25 millimeters.
 4. The heat transfer deviceof claim 1, wherein the difference between said first constant radius ofcurvature and said second constant radius of curvature is in the rangeof 10 to 2000 micrometers.
 5. The heat transfer device of claim 1,wherein said liquid coolant is a liquid metal and wherein said means forflowing said liquid coolant in said azimuthal direction comprise an MHDdrive comprising: a) a permanent magnet generating a magnetic fieldwithin at least a portion of said liquid coolant; said magnetic fieldhaving a substantial component in a radial direction of said hollowcylinder; and b) a plurality of electrodes for drawing electric currentthrough at least a portion of said liquid coolant in a directionsubstantially parallel to said central axis of symmetry.
 6. The heattransfer device of claim 1, wherein said liquid coolant is a liquidmetal and wherein said means for flowing said liquid coolant in saidazimuthal direction comprise a plurality of electromagnets; saidelectromagnets adapted for generating a magnetic field rotating in saidazimuthal direction in response to excitation by poly-phase alternatingcurrent.
 7. The heat transfer device of claim 1, wherein said means forflowing said liquid coolant in said azimuthal direction comprise onimpeller; said impeller arranged to form at least a portion of saidinner cylindrical surface; and said impeller arranged to rotate withrespect to said body; and said impeller arranged to substantially rotateabout said central axis of symmetry.
 8. A heat transfer devicecomprising: a body, an impeller, and liquid coolant; a) said body havinga first surface, a second surface; (i) said first surface being arrangedto be in a good thermal communication with a heat generating component;(ii) said second surface being arranged to be to be in a good thermalcommunication with a heat sink; b) said impeller being rotatablysuspended inside said body; c) said impeller and said body beingarranged to form a chamber; (i) said chamber being formed as a hollowcylinder comprising an outer cylindrical surface and an innercylindrical surface; (ii) said outer cylindrical surface comprising afirst constant radius of curvature; (iii) said inner cylindrical surfacecomprising a second constant radius of curvature (iv) said outercylindrical surface of said chamber being substantially formed by saidbody; (v) said inner cylindrical surface of said chamber beingsubstantially formed by said impeller; (vi) said inner cylindricalsurface comprising an axis of rotational symmetry; (vii) said chamberbeing substantially filled by said liquid coolant; and d) said impellerarranged to rotate substantially about said axis of rotational symmetry.9. The heat transfer device of claim 8, wherein said first surface beinggenerally tangential to said outer cylindrical surface with only a smallseparation between the two.
 10. The heat transfer device of claim 8,wherein the difference between said first constant radius of curvatureand said second constant radius of curvature is less than about 2,000micrometers.
 11. The heat transfer device of claim 8, further comprisinga means to rotate said impeller about said axis of rotational symmetry.12. The heat transfer device of claim 8, further comprising a pluralityof electromagnets fed with poly-phase alternating current; saidelectromagnets being arranged to generate an electromagnetic fieldrotating substantially about said axis of rotational symmetry; saidelectromagnetic field being arranged to operatively couple to saidimpeller; said electromagnetic field arranged to rotate said impellersubstantially about said axis of rotational symmetry.
 13. The heattransfer device of claim 12, further comprising a permanent magnet; saidpermanent magnet being mechanically coupled to said impeller; and saidpermanent magnet being arranged to operatively couple to saidelectromagnetic field.
 14. The heat transfer device of claim 12, furthercomprising an electromagnetic coil; said coil being mechanically coupledto said impeller; and said coil being arranged to operatively couple tosaid electromagnetic field.
 15. A method for transferring heat from aheat generating component to a heat sink comprising the steps of: a)providing a body, an impeller, and liquid coolant; said body comprisinga first surface and a second surface; said impeller rotatably installedin said body; said impeller and said body arranged to form together achamber; said chamber shaped generally as a hollow cylinder; saidchamber being substantially filled with said liquid coolant; b) rotatingsaid impeller; c) causing said liquid coolant to flow substantiallyazimuthally inside said cylindrical chamber; d) transferring heat from aheat generating component into said liquid coolant; e) transporting heatin said liquid coolant; and f) transferring heat from said liquidcoolant to a heat sink.
 16. The method for transferring heat of claim15, wherein said impeller substantially forms an inner cylindricalsurface of said hollow cylinder.
 17. The method for transferring heat ofclaim 15, further comprising the steps of: a) providing a heatgenerating component arranged to be in a good thermal communicationswith said body; and b) providing a heat sink arranged to be in a goodthermal communications with said body.
 18. The method for transferringheat of claim 15, further comprising the step of providing a means forrotating said impeller.
 19. The method for transferring heat of claim18, wherein said a means for rotating said impeller comprise a pluralitya plurality of electromagnets adapted for operation with poly-phasealternating current.
 20. The method for transferring heat of claim 18,wherein said a means for rotating said impeller are selected from thegroup of electric motor, rotating magnetic field, hydraulic motor, and aturbine.